This Essay, in honor of Oscar Schachter, discusses Thucydides’ account of the Peloponnesian War, not only glimpsing into the events surrounding the conflict but also considering how the sparring Greek city-states understood and manifested laws of war. This article describes numerous customs, practices, and procedures including respect for truces, ambassadors, heralds, trophies, and various forms of neutrality the ancients adhered to during times of conflict. The Greek city-states and their warriors recognized and enforced obligations concerning a city-state’s right to war (jus ad bellum) and conduct in war (jus in bello). While the ancients’ laws of war were always recorded in treaty, many of the laws were mutually recognized and formed out...